Support OS version in platform string #5769

Job	Run time
build (windows/amd64)	1m 55s
sandbox-build (linux/amd64)	6m 39s
build (freebsd/amd64)	1m 41s
sandbox-build (linux/arm64)	7m 0s
test-freebsd-amd64	2m 29s
test-windows-amd64 (containerd, ./client)	6m 49s
test-windows-amd64 (containerd, ./cmd/buildctl)	2m 52s
test-windows-amd64 (containerd, ./worker/containerd)	3m 2s
test-windows-amd64 (containerd, ./solver)	2m 46s
test-windows-amd64 (containerd, ./frontend)	3m 18s
test-windows-amd64 (containerd, ./frontend/dockerfile)	40m 35s
test-windows-amd64 (./..., 1)	4m 32s
	1h 23m 38s

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